The present invention relates to an image forming apparatus, such as a copying apparatus or the like, and more particularly to an image forming apparatus having a plurality of image forming modes such as, in the copying apparatus, a normal copying mode, a margin shift copying mode, a variable magnification copying mode and the like.
There have been conventionally used a variety of image forming apparatus for recording an image on paper. In an electrophotographic copying apparatus for example, copy paper is successively fed from a copy paper housing unit at predetermined time intervals, and delivered to registration rollers disposed in the vicinity of a transfer unit for transferring a toner image formed on the surface of a photoreceptor to the copy paper. The registration rollers cause the paper to be stopped. Thereafter, the paper is fed to the transfer unit with predetermined timing in synchronism with the rotation of the toner image on the photoreceptor.
Recently, copying apparatus is apt to be made in a larger scale and to be operated at a higher speed, thereby to lengthen the paper delivery passage from the paper housing unit to the registration rollers. Accordingly, when continuously carrying out a copying operation on a plurality of paper sheets, there are instances where a plurality of paper sheets are present in the delivery passage. Therefore, if the period of time during which each copy paper is stopped by the registration rollers, is changed, this may cause paper sheets to come into collision with each other or to be jammed in the delivery passage. Thus, the period of time that the paper is stopped cannot be changed while copy paper sheets are present in the delivery passage.
However, when the copying mode is changed, it becomes necessary to change the relationship between the exposure starting time of the photoreceptor and the drive starting time of the registration rollers. For example, in a margin shift copying mode for providing a margin at the left or right end of copy paper, it is required to advance or delay the drive starting time of the registration rollers according to the required margin width. On the other hand, in the enlargement copying mode or the like where it is required to operate the optical system at a lower speed with respect to the paper delivery speed, the time required for the optical system to reach the document read starting position after the optical system has started operating is lengthened. This causes the document read starting time, to be delayed. It is therefore required that the drive starting time of the registration rollers be delayed according to such a delay. However, the paper stop time cannot be changed while paper sheets are present in the paper delivery passage. This presents the problem that the copying mode cannot be changed.
An example of prior art for overcoming the problem above-mentioned is disclosed in, for example, JP-A-111651/1984. The paper feed control technique proposed in the document above-mentioned is so arranged as to change the time at which copy paper is taken out from the paper housing unit, based on the change in drive starting time of the registration rollers.
In the prior art proposed in the document above-mentioned, even though the time at which copy paper is taken out from the paper housing unit can be changed based on the change in drive starting time of the registration rollers, it is not possible to change the time at which the already taken copy paper, present in the paper delivery passage between the paper housing unit and the registration rollers, is delivered to the registration rollers. Accordingly, even in the prior art above-mentioned, the period of time during which copy paper is stopped by the registration rollers is temporarily changed when the copying mode is changed. This results in the likelihood that paper sheets come into collision with each other or are jammed in the paper delivery passage.